1. Field of the Invention
The present invention relates to a method for tracking the deterioration of the insulators in a rotating machine, a method for preventive maintenance of these machines using this tracking and devices using the tracking method.
2. Related Art
The manufacturers and users of rotating machines are often confronted by the behavior over time of the insulators that are present in the stator and the rotor (when they are wound) of these items of equipment. The latter create conditions promoting the appearance of partial electrical discharges in or in the vicinity of the insulators and the latter can in return contribute to the damaging of the insulators. All this in the end can cause electric failures leading to operating shutdowns, penalties or indirect expenses, costs of rewinding the stators and the rotors, or even the partial or complete replacement of the equipment.
Today, there is a certain number of solutions for monitoring the phenomenon of partial discharges and/or the state of the insulators. It is possible to cite in particular:                the electrical measurement of the partial discharges by capacitive coupler, which is the most widespread. It is based on measuring current transients in the windings due to the partial discharges. Usually, the method uses couplers installed in the motors, which couplers measure the frequency and the charge corresponding to these transient currents. This method makes it possible to evaluate the level of partial discharges in the stators of the rotating machines without it being necessary to shut the machine down. The data obtained by this method require interpretation and therefore a good knowledge of the partial discharge phenomena. It gives only an evaluation of the number and the amplitude of the partial discharges, but no indication of their impact on the insulation.        the measurement called the insulation resistance measurement: this measurement is taken with the motor shut down. It consists in applying a direct-current voltage between the conductor windings and the metal ground of the stator or of the rotor and in measuring the resistance of the electrical insulation between these two elements. Ideally this resistance is infinite but in practice it is not. The weaker the insulation resistance, the greater the likelihood of having an insulation problem. This measurement cannot be taken when the motor is operating and is extremely sensitive to temperature.        the measurement of the polarization index: this is a variant of the measurement of the insulation resistance which consists in computing the ratio between the insulation resistance after having applied the voltage to the insulator for ten minutes and the resistance obtained after one minute. A low polarization index usually indicates contamination of the insulator (oil, grease, etc.). This measurement is less sensitive to temperature than the insulation resistance measurement.        the Hipot test: it consists in applying a high direct-current voltage to the windings. If there is a defect in the insulation, it will cause, because of the voltage applied, a break in the insulation at this point. This test can be carried out only with the motor shut down and can be destructive.        the detection of certain types of gases created by the ionization of the air, a consequence of the corona or surface discharges. These methods relate mainly to ozone and NOx.        the measurement of the Ohm's law difference.        the measurement of the tangent delta.        the Terase criterion.        
All these methods have a certain value, but they require either the systematic shutting down of the motor, or an awkward interpretation of the partial discharge phenomenon in terms of impact on the insulators. This interpretation is often hazardous, because the characterization of the partial discharges is only indirectly linked to the state of the insulators.
It is possible to cite also other methods which make it possible to characterize partial discharges, but which do not seem to be suitable for the situation of rotating machines:                measurement of the thermal wave, using space charges trapped in the insulator caused by various phenomena (ionization, polarization, etc.). The method consists in applying a temperature difference which induces a current dependent on the electric field originating from these space charges. It is usually applied to insulated electric cables.        acoustic measurement, which detects the ultrasound emissions produced at the time of the discharges. The method is particularly suitable for transformers.        